1. Field of the Invention
The present invention relates generally to a system and method for eliminating short circuits caused by the production of dendrites in a fuel cell, battery or metal recovery apparatus.
2. Related Art
One of the more promising alternatives to conventional power sources in existence today is the metal/air fuel cell. These fuel cells have tremendous potential because they are efficient, environmentally safe and completely renewable. Metal/air fuel cells can be used for both stationary and mobile applications, and are especially suitable for use in all types of electric vehicles.
Metal/air fuel cells and batteries produce electricity by electrochemically combining metal with oxygen from the air. Zinc, Iron, Lithium, and Aluminum are some of the metals that can be used. Oxidants other than air, such as pure oxygen, bromine, or hydrogen peroxide can also be used. Zinc/air fuel cells and batteries produce electricity by the same electrochemical processes. But zinc/air fuel cells are not discarded like primary batteries. They are not slowly recharged like secondary batteries, nor are they rebuilt like "mechanically recharged" batteries. Instead, zinc/air fuel cells are conveniently refueled in minutes or seconds by adding additional zinc when necessary. Further, the zinc used to generate electricity is completely recoverable and reusable.
The zinc/air fuel cell is expected to displace lead-acid batteries where higher specific energies are required and/or rapid recharging is desired. Further, the zinc/air fuel cell is expected to displace internal combustion engines where zero emissions, quiet operation, and/or lower maintenance costs are important.
In one example embodiment, the zinc "fuel" is in the form of particles. Zinc is consumed and releases electrons to drive a load (the anodic part of the electrochemical process), and oxygen from ambient air accepts electrons from the load (the cathodic part).
The overall chemical reaction produces zinc oxide, a non-toxic white powder. When all or part of the zinc has been consumed and hence transformed into zinc oxide, the fuel cell can be refueled by removing the reaction product and adding fresh zinc particles and electrolyte.
The zinc oxide (ZnO) product is typically reprocessed into zinc particles and oxygen in a separate, stand-alone recycling unit using electrolysis. The whole process is a closed cycle for zinc and oxygen, which can be recycled indefinitely.
In general, a zinc/air fuel cell system comprises two principal components: the fuel cell itself and a zinc recovery apparatus. The recovery apparatus is generally stationary and serves to supply the fuel cell with zinc particles, remove the zinc oxide, and convert it back into zinc metal fuel particles. A metal recovery apparatus may also be used to recover zinc, copper, or other metals from solution for any other purpose.
The benefits of zinc/air fuel cell technology over rechargeable batteries such as lead-acid batteries are numerous. These benefits include very high specific energies, high energy densities, and the de-coupling of energy and power densities. Further, these systems provide rapid on-site refueling that requires only a standard electrical supply. Still further, these systems provide longer life potentials, and the availability of a reliable and accurate measure of remaining energy at all times.
The benefits over internal combustion engines include zero emissions, quiet operation, lower maintenance costs, and higher specific energies. When replacing lead-acid batteries, zinc/air fuel cells can be used to extend the range of a vehicle or reduce the weight for increased payload capability and/or enhanced performance. The zinc/air fuel cell gives vehicle designers additional flexibility to distribute weight for optimizing vehicle dynamics.
Typical zinc/air fuel cell stacks comprise a plurality of cells electrically coupled together serially. During operation of a zinc/air fuel cell stack, a supply of electrolyte is generally pumped through the cells continuously. This can also be useful in a zinc/air battery or in a zinc recovery apparatus. Electrolyte is typically pumped through the fuel cell stack using an input and output manifold device having a number of channels equal to the number of cells comprising the fuel cell stack. This parallel connection causes the formation of dendrites in the channels which can short circuit the fuel cell stack and block electrolyte flow.
A related dendrite formation problem has been addressed in U.S. Pat. No. 3,773,561 to Bjorkman. However, this patent proposes a solution to prevent the growth of dendrites and short-circuiting through electrolyte channels when a battery or fuel cell is shut down, and no electrolyte is flowing through the system.
Thus, what is needed is a system and method for eliminating short circuits and electrolyte channel blockage caused by the production of dendrites in an active fuel cell, battery or metal recovery apparatus, when electrolyte is flowing through the system.